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use of which is to afford point.centres der that article usually known by the when the points are turned towards each name of the foot-lathe. In the regular other, or hole-centres when the contrary foot-lathe, work is very seldom turned is the case; and lastly, there is a small between the opposite centres, though reșt, with its support, slidable and adjust this method certainly affords great truth able along the bar, as in another lathe. and precision. The mandrel is here an These instruments, which cost five or six essential part of the apparatus, which is shiliings at the watch tool shops, will always used has been shown, that it is therefore support any piece of four or supported by a centre on the left hand, five inches long, and three inches diame- called the back centre, and by a steel ter, between the centres, and the method collar in the middle poppet-head; and of producing the rotation is by passing that the right hand extremity, or nose of the catgut string of a bow once or twice the mandrel, terminates in a screw, eiround the work, and drawing the bow ther convex or concave, the latter of backwards and forwards with one hand, which is preferred in the best lathes. while the other is employed in applying the various descriptions of pieces screwthe tool. The turn-bench itself is held ed upon the nose of the mandrel, for steady in a vice fixed to a bench or holding or carrying work, are called stand.

chucks, probably, because the work is Such pieces as have a hole through the mostly fastened by being driven, jammed centre are drawn tightly upon an arbor or choked into them. or mandrel, having a pulley or ferril fix. When work is to be turned between ed upon it, to carry the gut or bow-string, centres by the foot-lathe, a centre-chuck, and the mandrel itself is turned between or steel-piece, carrying a projecting the centres upon its own pointed extre- point, is screwed on the nose of the manmities. There are mandrels fitted up in drel; and as this piece is not harder than different ways for holding the work firm- blue, and may not always screw home to ly, and if fat, at right angles to the mo- exactly the same bearing, accurate worktion; but we cannot, consistently with men are in the habit of turning or shav. brevity, enter upon a description of them, ing the point in its place, so that it shall which will immediately be understood by be truly centred. The opposite centre inspection in a workshop.

is afforded by the moveable poppet-head, The common lathe of the turners in ought to be truly in the axis; and wood, called the pole-lathe, is the same the mandrel is made to carry the work thing as the watchmakers' turn-bench, round by an arm and pin, or by any other but upon a large scale, and a little varied. ready method of connection, Instead of the horizontal bar, it has two Work, which is not to be turned belong stout bars of wood, called sheers, tween centres, is usually fastened to, or forming what is called the bed of the fixed in, a block or wooden chuck screwlathe, and its two poppet-heads are up- ed on the mandrel. As it would be alright blocks of wood, mortised in be most impossible to screw a wooden tween the sheers, above which they rise chuck upon the convex nose of a manand carry the centre screws, and between drel, and take it off as occasion required which they are moveable, and may be during the process, without altering the wedged firmly at any required distance position, it is found much best that the from each other. The work itself is ei. screw of the mandrel should be hollow, ther put between the centres, or upon a and a brass chuck screwed therein, hav. wooden mandrel, and it is made to re- ing its projecting screw to receive the volve by a string or band, proceeding wooden chuck ; because, by this means, from a long spinning pole at the ceiling the work may be taken off repeatedly, if or roof of the shop, round the work, and needful, without ever separating the thence to a treadle or foot-board, which brass and the wood; and the brass and acts by alternate pressure from the foot, the steel will take the same position while the workman applies the cutting when screwed together again. tool with his hands.

Metallic or other work may be fasten. In these, and all similar lathes, the ro- ed to a wooden chuck by cement, or by tation is made backwards and forwards; glue, or by turning a 'cell in the wood, nd there are some kinds of work in and driving the work gently and carefully whiclı such a motion is advantageous; but into it till fixed. in general it is much preferable that the The stronger, the firmér, and the bet. work should constantly revolve the same ter the workmanship of a lathe, the easiway, as shown in the lathe described un- er it will be to perform work with expedition and truth; but a good workman be cut extremely clean, a sharp hard tool will make true and excellent work with may be useful; but for the most part, in a very indifferent lathe, by taking care metallic work, even of steel, (if annealto cut so little at a time, that the parts of ed,) the hook tool, or graver, need not be the engine may never be shaken out of harder than purple, or even blue. But their contact. Metallic lathes, if ever so to cut steel work or chill cast iron cylin. strong, have an elastic tremor, which ders at a high temper, the tool must be makes it difficult to cut brass and belle very hard, the angle of edge obtuse, (say metal as firmly and smoothly as in wood seventy degrees,) and the motion slow. en lathes, but the structure of the former Hitherto we have spoken of plain turnadmits of greater precision and truth. ing, which is indeed the most useful and In a well constructed lathe, the back cen- most universally practised. But many tre, the centre of the collar, and the fore other nice and very curious operations are centre, or centre of the moveable poppet. performed by this art. If the poppet. head, ought to be in one line, parallel to heads, supporting the mandrel, be made the bed or sheers. To prove this by trial, regularly to move from side to side, durset the moveable poppet-head as far to ing the rotation, or the rest be made to the right hand as possible, and screw a approach to, and recede from, the work, stick of wood into the nose of the man- any number of times in a turn, the cuts drel: into the middle of the right hand will not be circular, but undulating, inend of the stick, or nearly so, drive a dented or waved in any curve that may be pin or other projecting point, and, by gen. required. Work of this kind, which is ile blows against the stick, cause the chiefly done in watch cases, snuff boxes, point to remain steady in the axis while and trinkets, is called rose-work. The the mandrel is turned round. If the cen. motion is commonly regulated by certain tre point of the moveable poppet be truly round plates of brass fixed on the manopposite to the revolving point, the three drel, called roves, which have their edges centres are in a line ; and if the same con- waved, and are called roses. tinues to be the case when the face of the Another deviation from regular turning moveable poppet is reversed, it is a proof is effected by causing the chuck, which that the hole in the poppet is bored pa- carries the work, to recede crosswise rallel to the bed : and if the same adjust. from the centre of the mandrel, back and ment continues when the stick is shorten- forward, during the rotation. The effect ed, it shows that the bed is straight, and of this is, that the diameters of the work parallel to the axis of work. If the col. are not all equal to each other. It is lar and back centre, and the chamfer and practicable to produce a variety of point of the mandrel, in a lathe, be truly curves in this way, but in our art the proformed and set square, the rotation cess is confined to turning ovals; and the slowly made by hand, when the back chuck, by which the work is made thus centre is rather firmly set up, will be to slide back and forward, is called an oval equally stiff in every part, and the wear- chuck. ing parts, when examined, will have the Numerous geometrical figures are pro. same aspect, slope, and grain, in every duced by turning, by an apparatus upon part of their surfaces.

the principle of the geometrical pen of The velocity of rotation may be ex. Suardi, in engines which have been made tremely swift in wood, slower in brass for curiosity, and at great expense. and bell-metal, still slower in cast iron, Medallions, and other similar pieces, and slowest of all in forged iron or steel. are produced by regulating the action of The reason for these limits appears to be, the tool in its advance to, or recess from that a certain time is requisite for the act the face of a piece exposed to its acof cutting to take place, and that the tool tion. itself, if heated by rotation, will instantly If the mandrel of a lathe be made to become soft, and cease to cut. Steel and advance and recede in the line of the axiron require to be kept wetted. For is, once in each turn, the cut will not be rough work in wood the gauge is a good in a plane at right angles to the axis of tool, and after that the chissel, with its the work, and the line traced upon the edge a little convex, rather than straight work will be an ellipsis, produced by the lined. The graver is commonly used for oblique section of a cylinder. This kind metal; and for strong rough work, the of work is called swash-work, and may hook tool, wlich is of excellent advan- been seen in some old balustrade, where tage, even in small work, on account of its effect is far from being pleasing. The its extreme steadiness. When steel is to nature of the curve thus described, sich we have called an ellipsis, will manifestly rately by Duhamel and others, as prac. vary according to the law of the alternate tised in the south of France. The fir is motion in the mandrel. When the man. generally allowed to remain untouched drel moves uniformly forward, the cut till it is thirty or forty years old. When will be the common helix or screw;

and

it is to be worked, which is early in the the motion is used to make screws, spring, a small hole is first made in the though not very frequently, because good ground at the foot of the tree, the earth turners can easily make them by a notch- of which is well rammed, and serves as a ed cutting tool, called the screw,

receptacle for the juice. The coarse The act of turning is so extensively ap- bark is then stripped off from the tree, a plicable, that it would require a volume little above the hole, down to the smooth to describe its uses, and the methods of inner bark, after which a portion of the practising it. Every round thing which inner bark, together with a little of the is made by human hands may be referred wood, is cut out with a very sharp tool, to this art, as one of its products. The so that there may be a wound in the tree largest columns, the most ponderous ar- about three inches square, and an inch tillery, and the minutest pivots of watch- deep. Immediately afterwards the tur. work, with all wheel-work, rotatory ma- pentine begins to exude in very transpachines, vessels, &c. are worked in this rent drops, which escape chiefly from the method.

wood immediately under the inner bark. TURNSOLE. See LITMUS.

The hotter the weather is, the greater is TURPENTINE. See RESINS,

the supply of resin; and to facilitate the Turpentine, of which there are various supply, the incisions are enlarged every kinds, are all products of some of the three or four days, by cutting off thin species of the pinus. From this genus are slices, till at the end of the year it is about obtained not only turpentine, buť rosin, a foot and a half wide, and two or three pitch, tar, &c. which are employed so ex- inches deep. The whole time during tensively in ship-building, and in the rig. which the turpentine Aows is from the ging also : likewise in varnishes.

end of February to October. In the winThere are three varieties of pine tur- ter it entirely ceases, but in the ensuing pentine, commonly known under that spring a fresh incision is begun a little name in Europe: namely, 1. The com- above the former, and managed in the mon turpentine, obtained chiefly from the same manner. This practice is continued pinus sylvestris (Scotch fir). 2. The annually for about twelve or fifteen years Strasburgh turpentine, yielded by the pic in some parts, and in others a shorter nus picca (silver fir)." And, 3. The Ve. time, on the same side of the tree, till the nice turpentine, procured from the pinus later incisions are so high as to be out of larix (larch). Of the three first mention. reach without the assistance of steps; af. ed turpentines, the Venice is the thinnest ter which the contrary side of the tree is and most aromatic; the Strasburgh the begun upon, and worked in a similar mannext in these qualities; and the common ner for as many years, during which is the firmest and coarsest. The two time the first incisions are grown up, and former are often adulterated by a mixture are fit to be cut afresh. In this way, a of the common turpentine and oil of tur- healthy tree, in a favourable soil, may be pentine; and it is to be observed, that the made to yield from six to twelve, or more, terms Venice and Strasburgh turpentine pounds of turpentine annually, sometimes are not now appropriate, as they are pro- for a century; and even the timber is not cured from various countries.

soon injured by this constant drain. The Common turpentine is obtained largely Aow ofturpentine discontinues altogether in the pine forests in the south of France, about October, and the liquid resin colin Switzerland, in the countries on the lected during the year, from each tree, is north of the Pyrenees, in Germany, and put together for further purification. But in many of the southern states of North a considerable quantity of the resin has America. The greater part of what is concreted during that time around the in. consumed in this country is imported cision, particularly as the heat declines; from North America. The method of and in the winter, when it has hardened obtaining it is by making a series of in- considerably, it is scraped off, and forms cisions through the bark of the tree, from what is technically called barras, or in which the turpentine exudes, and falls some provinces galipot, which differs down into boles, or other receptacles at from the more liquid turpentine in conthe foot.

sistence, and probably contains a less proThe process is described very accui- portion of essential oil. The galipot is much used in making flambeaux, when Turquoise is generally considered as mixed with suet; but the greater part of fossil-bone, or ivory penetrated by oxide it, as well as the liquid turpentine, is sub- of copper; it appears, however, from the jected to further processes.

above analysis, that the colouring matter The Strasburgh turpentine, the produce is phosphate of iron. The oriental turof the silver fir, is the most fragrant of all quoises are found near Meched in Persia, the pine turpentines, and only inferior to also in Mount Caucasus, in Egypt and the true Chio; but it is not often seen in Arabia. The occidental ones are found the shops. It is obtained by rude incision in Languedoc in France, and in Hungary. of the bark by the peasants in the vast Turquoise was formerly in some estima. pine forests on the western Alps. The tion for rings and other articles of per. first cut is made as high as the hatchet sonal ornament; but its value has greatly will reach, and these are renewed annual. declined in modern times. The colour ly from above downwards to within a foot turquoise changes gradually by exposure of the ground. But the finest kind of tur- to the air, from blue to green : when it pentine yielded by this tree is that which arrives at this state, its commercial value exudes from soft tubercles, or swellings is wholly extinct. of the inner bark. The peasants carry TURRITIS, in botany, tower-mustard, a with them a large cow's horn, with the genus of the Tetradynamia Siliquosa point of which they pierce these tuber. class and order. Natural order of Silicles, and collect the juice in its hollow. quosæ, Cruciformes, or Cruciferæ. Essen

The true Venice turpentine, or resin of tial character: silique very long, angular ; the larch, is obtained from the Tyrol and calys converging, erect; corolla erect. Savoy, and also from Dauphiny, by boring There are eight species. holes about an inch in diameter, with a TURRÆA, in botany, a genus of the gentle descent, in the most knotty parts Decandria Monogynia class and order. of the tree. To these are adapted long Natural order of Trihilatæ. Meliæ, Jusperforated pegs, which serve as gutters sieu. Essential character: calyx fiveto convey the juice into troughs placed toothed; petals five; nectary toothed, beneath. It is yielded during the whole cylindrical, bearing the anthers at the of the summer, and is simply purified by mouth between the teeth; capsule penstraining through hair sieves. A full tacoccous; seeds two. There are five grown larch will sometimes yield seven species. or eight pounds of turpentine annually TUSCAN order, in architecture, the for forty or fifty years.

first, simplest, and most massive of the TURQUOISE. The colour of this suh. five orders. stance is pale sky-blue, passing into indi- TUSSILAGO, in botany, colt’s-foot, a go-blue, and pale apple-green. It occurs genus of the Syngenesia Polygamia Su. in mass, or disseminated. Its fracture is perflua class and order. Natural order

Its hardness is nearly equal to of Compositæ Discoideæ. Corymbiferæ, that of glass; it is difficultly frangible. Jussieu. Essential character: calyx scales Specific gravity 3.12. Before the blow- equal, as long as the disk, some what pipe its colour changes to greyish-white, membranaceous; down simple ; recepta. and it becomes friable, but it does not cle naked. There are fourteen species. melt. It is soluble in nitro-muriatic acid,

TWILIGHT, that light, whether in the and the European varieties are so in nitric morning before sunrise, or in the evenacid; this menstruum, however, has no ing after sunset, supposed to begin and action on the Persian turquoises. It is end when the least stars that can be seen composed, according to Buillon la Grange, by the naked eye cease, or begin to apof

pear. By means of the atmosphere it

happens, that though none of the sun's Phosphate of lime

80

direct rays can come to us after it is set, Carbonate of lime

8

yet we still enjoy its reflected light for Phosphate of iron, with a

2 some time, and night approaches by detrace of manganese

grees; for after the sun is hidden from Phosphate of magnesia

our eyes, the upper part of our atmoAlumina

1

sphere remains for some time exposed to Water

6

its rays, and from thence the whole is ilLoss

luminated by reflection. But as the sun

grows lower and lower, that portion of 100

the atmosphere which is above our hori. zon becomes enlightened till the sun has

even.

1

Latitude.

o

10

20

30

40

45

50

got eighteen degrees below it; after tinual night; and w 11, that the twilight which it ceases to be illuminated thereby, lasts the whole night: till it has got within as many degrees of the eastern side of the horizon; at which time it begins to illuminate the atmosphere again, and in appearance to dif

**SOCO fuse its light throughout the heavens, which continues to increase till the sun

269 be up. Hence it is, that during that part of the year in which the sun is never eighteen degrees below our horizon, there is a continued twilight from sunsetting to sun-rising. Now that part of the year in the latitude of London is, while the sun is passing from about the fifth degree of Gemini to the twelfth of Cancer; that is, from the middle of May to the middle of July.

As the twilight depends on the quantity of matter in the atmosphere fit to reflect the sun's rays, and also on the height of it, (for the higher the atmosphere is, the longer will it be before the upper parts of it will cease to be illuminated,) the duration of it will be various. For instance, in winter, when the air is condensed with cold, and the atmosphere upon that account lower, the twilight will be shorter; and in summer, when the limits of the atmosphere are extended by the rarefaction and dilatation of the air of ৩ ৩ ১৩ ১৩ ১৩ ৪ ও ৮ which it consists, the duration of the Econo twilight will be greater. And for the like reason, the morning twilight, the air being at that time condensed and contracted by the cold of the preceding night, will be shorter than the evening one, when the air is more dilated and expanded.

couco The beginning and end of twilight has been variously stated, by different ob. servers; but, in our latitude, it may be said to begin and end when the sun is about eighteen degrees below the horizon: hence, when refraction is allowed for, the atmosphere must be capable of reflecting sensible light at the height of about forty miles. The duration of twi. light is greater or less as the sun moves more or less obliquely with respect to the horizon: hence it is shortest near the time of the equinoxes, because the equinoxial intersects the horizon less oblique. ly than any lesser circle parallel to it. Dr. Long has calculated the duration of twilight in different latitudes, and for the several different declinations of the sun: TYCHONIC system, or hypothesis, an or. the result he laid before the public in the der or arrangement of the heavenly bofollowing table, where the letters c d sig- dies, of an intermediate nature between nify that it is then continual day; cn con- the Copernican and Ptolemaic, or parti

1 18 1 19 1 23) 1 30 1 43 1 531 2 6 2 15 2 26 2 57 4 4 10 24 9 301 7 46
i 16 i 17 i 21 i 28 1 40 1 49 2 1 2 8 2 18 2 43 3 2611 38 11 14 10 32 8
1 13 1 14 1 18 1 24 1 35 1 43 1 54 2 0 2 8 2 27 2 56 8 41 5 2 17 32
i 12 i 13 i 17 i 24 i 35 1 44 1 55 2 2 2 10 2 33 3 8 4 18 w nw nwn
1 13 1 15 1 20 1 28 1 43 1 55 ? 12 2 25 2 41 3 55 w nwo wnc ac d
1 16 1 19 1 25 1 36 1 58 2 19 3 3 w ni w nw nwnc dc dc d c d

18 1 21 1 28 1 41 2 8 2 39 w nw nw
Enters/ h. m h. m. h. m h. m. h. m. h. m. h. m. h. m. h. m. h. m.

n w nw n c d c d c d c d

h.m h. m.) h. m. h. m h.m.h.m

TABLE.

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